Total body perfusion by pump oxygenator still causes measurable and important damage to blood elements. The pathological changes range from disruption or aggregation of cellular elements to alterations in the coagulation system, and they are important in the morbidity and mortality of open heart surgery. We propose to study the quantitative changes in the cellular and chemical constituents of blood during perfusion in humans, in order to identify the sources of damage in detail and find means to eliminate them. Particular, but not exclusive, attention will be paid to the changes which are caused by the extracardiac suction system.